Circuit for minimizing the effects of noise in television signals



March l0, 1953 C. O. MARSH, JR CIRCUIT FOR MINIMIZING THE EFFECTS OF NOISE IN TELEVISION SIGNALS Filed Oct. 1l. 1949 utilized to control a clipping level.

Patented Mar. 10, 1953 kCIRCUIT Fon MINIMIZING THE EFFECTS oF j Y NoIsE 1N TELEVISION SIGNALs Clee 0. Marsh, Jr., Nutley, N. J., assignor to Allen B. Du Mont Laboratories, Inc., Passaic, N. J., a corporation of Delaware Application October 11, 1949, Serial No.,120,712

'1 Claims. (o1. 25o-20) This invention relates to minimizing the effects of noise in radio signals, and more particularly to the removal of noise peaks from received television signals.

In the past, large peaks of interfering noises have been removed from desired signals by clipping. For best noise` reduction it is desirable that clipping occur close tothe maximum excursion of the desired signal. Toprevent Vdistortion of Athe desired signal or removal of synchronization signals it is necessary that the clipping level does not occur within the desired signal.

An object of my invention is to provide a clipping tube and circuit in whichclipping is automatically maintained at the proper level for a wide range in amplitude of the incoming signals.

Another object is to providel apparatus which in conjunction with an automatically controlled clipping tube is adapted to minimize the effects of noise impulses when they'are mixed with a received television signal. Y

`Another object is to provide a method by which the magnitude of noise mixed with a desired signal may be reduced. Y

In accordance with the invention,the current owing through a gain controlled amplifier is Thereafter the signal is passed through a narrow band coupling network and again clippedV in a sync separating circuit.

In the drawings:

through a video frequency filter 23 to therinput electrode of a clipping tube 24. The plate or output electrode of the clipping tube 24 is coupled to a video filter 26 and to a video amplifier 28 of the usual type. The output of the video amplifier 28 is connected to a beam intensity controlling electrode of a cathode ray picture tube 29. A load resistor 3| connected to the video iilter` 26 provides a load forsynchronization signals obtained through the plate circuit of the clipping tube 24. A coupling capacitor 33 connects the load resistor 3| to a grid leak resistor 34 and a series resistor 35, the latter resistor Vligurell shows a television receiver partly in block form and partly schematic;

Figure'Z'shows the mutual characteristics of a clipping'tube for different applied screen voltages; I

vision signal occurs in the usual amplifier stages means of a capacitance I3 to a second intermediate-frequency amplifier tube I5. The plate of this latter tube I5 is coupled by means of anintermediate frequency band pass illter I6 to a third intermediate frequency amplifier tube I8. The plate of this third tube I8 is coupled by means of a second intermediate frequency band pass lter I9 to the cathode of a video detector The demodulated signal from the plate of the video detector tube 2l is directly coupled being connected to the grid of a synchronization signal separator tube 31. The plate of the separator tube 31 is kconnected to and provides signals to horizontal and vertical deflection circuits 38 of the usual type.

The video lter 23, connected between the video detector 2l and the grid of the clipping tube 24, is connected to a detector load resistor 4I in which video voltages are provided by virtue of currents flowing through the detector tube 2'I. Intermediate frequency components of the detected signal are bypassed through the capacitances-to-ground of the tubes and lter elements. To this load resistor 4I are connected in series an automatic gain control resistor 42 and a capacitor 43, which two elements cooperate to remove vestiges of intermediate frequency from an automatic gain control bus 45 connected to the junction of the resistor 42 and the capacitor 43.V This latter bus 45 provides negative control grid bias for early amplifier stages I2 and for the second amplifier tube` tube I 5 in connected to the automatic gain control bus through the series combination'of a grid leak resistor 41, and a filter resistor 48, the junction of the resistors 41 and 48 being connected to and by-passed to ground by means of a filter capacitor 49.

In a broad sense the amplifier stages I2, I5, and I8, and the detector tube'2I may be considered to be bias-controlled ampliers connected to and supplying signals to K the clipping tube 24.

A source 5I of direct voltage supplies power for the receiver including the third intermediate frequency amplifier tube I8 and the plate or output electrode of the clipping tube v2li. A resistor 52 is connected between the power source 5I and the plate and the screen grid, which are anode elements of the second amplifier tube I5. The

, screen of theclipping. tube 24 is connected to the top of the resistor S21-through a screendropping through the resistor 52.

resistor 53. The anode of the amplifier tube I5 is connected to the top end of the resistor 52 through a portion of the bandpass rllter I6. This portion serves as a load impedance for the ampliiier tube I5. Thus the screen grid of the clipping tube 24 is connected through a common impedance 52 :Withvthe anode of the lampliier tube fI5to-th`e source o'f voltage. Screens of both tubes are bypassed to ground by means of individual bypass capacitors 55 and 56. r

The operation of the clipping tube can be un derstood by referring to Figure 2 .in-conjunction with Figure 1. If a large signalof for'instance 50,000 microvolts is receivedatthe antenna, it is producing a large signal at the detector. The large signal at the detector. 2l ,results vinthe .for-

mation of a large negative bias "on the'automa'ticgain control bus 45 which reduces thegain or I' -fedinto-Ithefclipper tube.l '"For'this condition the *second I. Ffamplifiertubel vis biased ofi so that only ai small current flowsY -througlfi` this tube and Thisresults in a high vv:screen voltage `for thessecondamplier tube I5, `raising the voltage of the screen 'dropping resistor 55s/and resulting'inf themutuai characteristic,

' `V`indicated at 58,I 'of the'I clipping tube 2lihaving a more-'negativecutoff levelidentiiied by numeral f5-9; to accommodate the large input signal.

i *Incomparison? to :the above described condition, Ywhen a fsmallersignal of for instance 200 Amicrovolts Vis received at the.-anteiina it is -amplis :ed'producing a smaller signalfGI at the plate offthedetectortube and producing a less 'negative bias voltage on the automatic gain control '."bus, resulting'in high 'amplifier gain. Because 'the"bias`ivoltag`e 'onthe second -amplier tube I5 isfless ne'gative'than before, more .current flows "through 'the screen-and -plate of this tube, pro- "fduciiig agreater 'voltage drop the resistor 52 :and-resultingin Aa lower screen voltage for ythe 'clipping tube'24. A `lower screen Voltage results in'a mutual-characteristicSZ having a less nega- .tive'cutoilev'el163.v clipping level both large'signals 51 and small signals 6I are clipped close to the desired level, eliminatinglas 'much' as possible of undesired noise 'peaks videntiiied by numeral B5, in the clipped-sig# @naisY shown. at 67 and 68.v The rst vclipping zilevelfmust bemaintainedcloseto the level ofthe a 'synchroniaationsignals.tor awide variety. of relf Aa source-of. positive .direct voltagelliaving' a ...high internaldmpedance. is .utilized-to-Supply d- -vrectrcurrent to both the bias. controlled amplier -andqthescreen .ci .the clipping tube, the-resistor t \"\2,may@be fomitted.- .Howevei', it Will then be necessary to have most of the uncontrolled portionsof `the set receive .current either 4from a separatesupply-or from@ .portion of the-supply v .having little impedance. .in common. with .the

high internal impedancepoition of .the supply.

In Figure 3 is shown the voltage obtained when the clipped current'i is Aimpressed upon the video lterl.. Noiseimpulses identiedby separate numerals occur in various parts of the de- Asired signals, an impulse II occurring during the synchronizing pulse, and impulse I2 occurring in @10j across the load resistor 3I.

As a resultof the change in Y 4 cursions due to noise do not exceed a certain positive level, so that no grid current is drawn in the video amplifier 28. If grid current were drawn in the video amplifier, the noise pulses 5 would cause trailing white to appear in the video tube. The trailing Whites if present, would cause muchmore visible noise in the picture .than short duration blacks caused by the noise itself.

In Figure 4 is shown the voltage appearing This resistor is effectively shunted by the tube capacitance of the clipping tubeand by the capacitance-to-ground f of the-coupling network 26 and the video amplifier ,-28. Because -of .this shunting effect, the voltage l5v across this resistor contains for the most part fre- Y. quencies below half a megacycle. This pass band l v'is considerably narrower than the pass band of approximately `lt'iiiegacycles amplified by the clipper tube 24 and by the video amplifier 28. As a result of the narrow band and the integrating ation, bisher .frequencies are remi/@dimm the vbltas'e Wave.y 1n particular; theamnltuies 0f theshort.duiationnoise pulseslI-'l are Vconsiderably reduced in `cornparisonto thosein a 25 Wide band network. lThe reduction of these short pulses is' approximately proportional. to

.the reduction in band width. j

The voltage .Wave of Figure 4 is coupled tothe grid of the synchronization signal separator t'ube 3 1', the values of the resistors 34jand 35 and the capacitorv 33 controlling the bias ofthe clipping ltube being such that double clipping Voccurs Within the region of the synchronization signal I5 itself. The double clipping operation which clips 35 -oii and removes vboth positive and negative eX- tremitiesof the signal, completely removes the .noise impulses I I,.'I2, and 13 from the synchroni- `zation signals,.1eavng only onenois'e peak I4 vWhichhappened to occur .in the black region of 40 the picture. A f

The wave form of the synchronization signal, after being successively clipped, narrowed, and -dou'bly clipped, is shown in Figure 5. The improvement in eliminating noise can-be seen by comparing it to the signals- 5'I'or 6I appearing at the detector. f y

`'Although a specific embodiment of the invention has been shown and described, the scope Y thereof is dened inthe following claims.

-What is claimed is: v

1. In a television receiver, -a-Wide band amplifier for video and synchronizing signals, said amplielincluding 'a -thermionic 'tube having yan anode, a clipper eircuit,a \diode connected-be- .tween lsaidclipper circuit andfsaidwide` band ',ainplier, said clipper circuit' includingza thermionic tube having a screen grid, a sourcegof-fvolt- Y age and v` a'resistance *connected thereto, the anode of said ampliiierand the screen grid of said clipper being connected `to the other end of said resistance. `In a televisionl receiver having an amplifier having an output electrode and the ygainth'ereof nbeing controlled by a bias voltage, acli-pping tube havinganeinput'electrode and ascreengrid;and

a high impedance` -fsource of positive potential comprising avoltage sourceand a resistance vcon- .nected in series therewith connected to provide .dir-ect. current to said output electrode and to saidscreen grid. n

Y 3. The apparatus ofclaim-1,I in which the con- .nectionof .saidscreen gridl to. said resistance .comprises a @second resistance 'and a bypass .capacitor. e

:4: erwin-@dirigerGirelli?1F01@ sleiriaiflsignais subject to noise interference, comprising an amplifier tube for said signa-ls and having an input electrode and an output electrode, a source of automatic gain control bias voltage connected to said input electrode, a. clipper tube having a control electrode and a screen grid, a detector tube connected between said control electrode of said clipper tube and said output electrode, and a. voltage source having a. resistance in series therewith connected to provide current to said output electrode and to said screen grid.

5. The circuit of claim 4, in which said automatic gain control bias voltage is derived from said detector tube.

6. The circuit of claim 4, including an isolating resistance connected in series with said screen grid between said screen grid and said output electrode and first-mentioned resistance.

'7. The circuit of claim 4, including a load impedance connected in series with said output electrode between said output electrode and said screen grid and rst-mentioned resistance.

CLEE O. MARSH, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

